zfsboot: fix build with WITHOUT_LOADER_GELI

[lwhsu]

Summary

cc -target x86_64-unknown-freebsd12.0 --sysroot=/usr/obj/usr/src/amd64.amd64/tmp -B/usr/obj/usr/src/amd64.amd64/tmp/usr/bin  -O2 -pipe   -I/usr/src/stand/i386/btx/lib -nostdinc -I/usr/obj/usr/src/amd64.amd64/stand/libsa32 -I/usr/src/stand/libsa -D_STANDALONE -I/usr/src/sys -Ddouble=jagged-little-pill -Dfloat=floaty-mcfloatface -DLOADER_DISK_SUPPORT -m32 -ffreestanding -mno-mmx -mno-sse -mno-avx -mno-avx2 -msoft-float -march=i386 -I. -DBOOTPROG=\"zfsboot\"  -O1  -DZFS -DBOOT2  -DSIOPRT=0x3f8  -DSIOFMT=0x3  -DSIOSPD=115200  -I/usr/src/stand/common  -I/usr/src/stand/i386/common  -I/usr/src/stand/i386  -I/usr/src/stand/libsa/zfs  -I/usr/src/sys/crypto/skein  -I/usr/src/sys/cddl/boot/zfs  -I/usr/src/stand/i386/boot2  -Wall -Waggregate-return -Wbad-function-cast -Wno-cast-align  -Wmissing-declarations -Wmissing-prototypes -Wnested-externs  -Wpointer-arith -Wshadow -Wstrict-prototypes -Wwrite-strings -g -MD  -MF.depend.zfsboot.o -MTzfsboot.o -std=gnu99 -Wsystem-headers -Werror -Wno-pointer-sign -Wno-empty-body -Wno-string-plus-int -Wno-unused-const-variable -Wno-tautological-compare -Wno-unused-value -Wno-parentheses-equality -Wno-unused-function -Wno-enum-conversion -Wno-unused-local-typedef -Wno-address-of-packed-member -Wno-switch -Wno-switch-enum -Wno-knr-promoted-parameter -Wno-parentheses  -Qunused-arguments  -c /usr/src/stand/i386/zfsboot/zfsboot.c -o zfsboot.o
/usr/src/stand/i386/zfsboot/zfsboot.c:711:11: error: no member named 'gdev' in 'struct zfsdsk'
    zdsk->gdev = NULL;
    ~~~~  ^
1 error generated.
*** Error code 1

Stop.
make[5]: stopped in /usr/src/stand/i386/zfsboot

Test Plan

build world and kernel with -DWITHOUT_LOADER_GELI

[• ian]

Ooops, sorry about that, I guess I never re-tested WITHOUT_LOADER_GELI after making various changes after the first rev.